Our objective is to establish the biochemical basis for radiation-induced mitotic delay. We are taking advantage of the natural synchrony of nuclear division in the plasmodial phase of Physarum polycephalum, in which we previously determined the times in late G2-early prophase when the plasmodium loses sensitivity to the induction of mitotic delay by gamma-radiation and/or cycloheximide and the effects on radiation-induced mitotic delay of compounds which alter cyclic nucleotide metabolism. The approach is (a) to determine the effect of gamma-radiation during late G2 and prophase on selected biochemical events which may be involved in the process of mitosis (e.g., cyclic nucleotide metabolism, histone phosphorylation, histone kinase activity) and (b) to correlate each of these effects with observable changes in nuclear cytology following radiation exposure. The correlation of physiological, biochemical, and cytological information may elucidate the mechanism by which ionizing radiation delays nuclear division.